Optoelectronic devices such as lasers, photodiodes and other photodetectors, have become widely used in the telecommunications and other industries. In optoelectronic devices, an electrical signal is converted to an optical signal that travels along an optical transmission medium such as an optical fiber, and is then converted back to an electrical signal. A high optical coupling efficiency is required to ensure good optoelectronic connections between the light source and the optical transmission medium, as well as between the optical transmission medium and the photodetector which detects the optical signal and converts the optical signal to an electrical signal.
In fiber-coupled packaging, laser light, which propagates through an optical fiber, is coupled into the active area of a photodetector either by a lens or by direct fiber coupling, depending on application. The optical coupling region typically includes an air gap between the optical fiber and photodetector and, when a lens is used, an air gap between the optical fiber and lens as well as between the lens and photodetector. The optical performance, or coupling efficiency, is limited by light loss due to reflection at the air/fiber, air/lens and air/photodetector interfaces. These effects are especially significant in 2.5-10 Gb/s (gigabits per second) applications because of the smaller active areas of photodetectors used in such applications. This makes it increasingly difficult to attain high optical performance or high optical coupling efficiencies in 2.5-10 Gb/s applications and, in turn, adversely affects the subsequent RF performance. It would therefore be desirable to provide an optical fiber coupled to a photodetector in which light loss due to reflection is eliminated or minimized. Previous attempts to address this issue include the use of various different lens types to improve focusing. This approach is limited by package size and the space available for positioning such a lens, especially in packages of reduced size such as used for high-speed applications. Furthermore, this approach does not address the loss in optical coupling efficiency due to light reflection at the air/photodetector interface.
In direct fiber coupling packaging, previous attempts to improve optical coupling efficiency include cleaving the optical fiber at an angle with respect to the photodetector, the angle selected to minimize back reflection. Another approach was tilting the photodetector at an angle with respect to the primary direction of the light beam being detected. Changing the cleave angle, however, only changes the direction of reflection to avoid laser light being directed back to the source. The loss of light still exists due to reflection at the interfaces between the angled end face of the optical fiber and air, as well as at the interface between the photodetector and air. Reflection also still exists when the photodetector is tilted and therefore the loss of light and reduced optical coupling efficiency still exists.
It would therefore be desirable to couple an optical transmission medium such as an optical fiber, to a photodetector, such that the amount of light lost between the optical fiber and photodetector is minimized or eliminated.